Basic Documentation

Elbow Airflow Noise
Determine the values for the terms in the Elbow Sound Power Level (DLw) formula:
ELw = K + 10 Log F + 50 Log U + 10 Log S + 10 Log D – EC
Where:
F = the octave band center frequency in Hz: (63, 125, 250, 500, 1,000, etc.).
U = the airflow velocity (12,000 cfm/6 sq ft = 2,000 fpm).
S = the duct area sq. ft. (36 in. x 24 in. =3 ft x 2 ft = 6 sq ft).
D = the vane cord length in feet (0.8 ft.).
For an elbow with turning vanes, EC = 10 Log n – 107 = 10( 1.18) - 107 = 11.8- 107 = -95.2.
K requires solving for the Strouhal number at each octave band center frequency and then
using the graph in Figure 12 to determine K.
St = 60FD÷ U St=60 x 63 Hz x0.8 ft.÷ 2,000 = 1.5, K = -48
St =60 x 125 Hz x 0.8 ft. ÷ 2,000 = 6.0, K = -52
St = 60 x 250 Hz x 0.8 ft. ÷ 2,000 = 12.0, K = -58
St = 60 x 500 Hz x 0.8 ft. ÷ 2,000 = 24.0, K = -65
St = 60 x 1,000 Hz x 0.8 ft. ÷ 2,000 = 48.0, K = -76
St = 60 x 2,000 Hz x 0.8ft. ÷ 2,000 = 96.0, K = -91
St = 60 x 4,000 Hz x 0.8 ft. ÷ 2,000 = 192.0, K = -111
St = 60 x 8,000Hz x 0.8 ft.÷ 2,000 = 384.0, K = -130
Note that after the first St number is calculated, the remaining seven values can be quickly
determined by just doubling the previous St value.
Table 9 summarizes the terms and results of using the ELw formula to determine the net
sound power level of this elbow at each octave band.
Table 9. Sound Calculation Summary for Actual Fan Operating Conditions.
63 125 250 500 1,000 2,000 4,000 8,000
Hz Hz Hz Hz Hz Hz Hz Hz
K -48 -51 -52 -58 -65 -76 -91 -111
10 Log F 18 21 24 27 30 33 36 39
50 log U 204 204 204 204 204 204 204 204
10 Log S 8 8 8 8 8 8 8 8
10 Log D -1 -1 -1 -1 -1 -1 -1 -1
EC -95 -95 -95 -95 -95 -95 -95 -95
ELw 86 86 90 85 81 73 61 44
dB dB dB dB dB dB dB dB
Siemens Building Technologies, Inc. 37